submodule--helper: fix obscure leak in module_add()

submodule--helper: fix obscure leak in module_add()

Fix an obscure leak in module_add(), if the "git add" command we were
piping to failed we'd fail to strbuf_release(&sb). This fixes a leak
introduced in a6226fd772b (submodule--helper: convert the bulk of
cmd_add() to C, 2021-08-10).

In fixing it move to a "goto cleanup" pattern, and since we need to
introduce a "ret" variable to do that let's also get rid of the
intermediate "exit_code" variable. The initialization to "-1" in
a6226fd772b has always been redundant, we'd only use the "exit_code"
value after assigning the return value of pipe_command() to it.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Reviewed-by: Glen Choo <redacted>
Signed-off-by: Junio C Hamano <redacted>